Earlier quoted context omitted.
How do you conclude that? It is my understanding that the Nixon tapes were recordings of conversations Nixon had with campaign staff as a candidate, and as the Supreme Court held, actions taken as a candidate are not official actions and are therefore not subject to immunity.
Nixon still had the presidency as he ran for reelection, allowing for the argument that tapes of anyone under the Whitehouse's employ were inadmissible official acts.
Law enforcement has qualified immunity for the vast majority of what they do in an official capacity. That doesn't mean their testimony about what they do in an official capacity is inadmissible, including if they testify about what other law enforcement officers did.